The 26-year-old flew to Italy early on deadline day and City announced the deal shortly before the 11pm cut-off point.
The England defender was not part of Manuel Pellegrini's squad for the season and last featured in the Premier League in November last year.
He becomes the second Englishman to move to Serie A this summer following Ashley Cole's decision to join Roma.
A City statement read: "Micah Richards has joined Serie A side Fiorentina until the end of the season.
"One of the most popular members of Manuel Pellegrini's squad, Micah has won 13 England caps to date and was an integral member of the City squad who won the title in 2011/12.
"A succession of injuries saw the Birmingham-born defender gradually fall behind Pablo Zabaleta in the first-team pecking order and the arrival of Bacary Sagna meant Pellegrini had three top-class right-backs at his disposal this season."
City skipper Vincent Kompany praised Richards on his exit, writing on Twitter: "#MCFC club legend! Proud to have played 6 years alongside this man. Congratulations and good luck at @ACF--Fiorentina my brother #forzaMicah."